This invention relates generally to data communications and, more particularly, to methods and systems for time domain equalization of data signals received from a data communications channel and for channel shortening.
Channel shortening can be thought of as a generalization of equalization, since equalization amounts to shortening the channel to length 1. Channel shortening was first utilized in an optimal estimation method that minimizes the error probability of a sequence, maximum likelihood sequence estimation (MLSE).
A form of channel shortening can also be utilized in multiuser detection. For a flat-fading DS-CDMA system with L users, the optimum multiuser detector is the MLSE detector; yet, complexity grows exponentially with the number of users. “Channel shortening” can be implemented to suppress L-K of the scalar channels and retain the other K channels, effectively reducing the number of users from L to K.
Channel shortening has recently seen a revival due to its use in multicarrier modulation (MCM). MCM techniques such as orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT) have been deployed in applications ranging from the wireless LAN standards IEEE 802.11a and HIPERLAN/2, Digital Audio Broadcast (DAB) and Digital Video Broadcast (DVB) in Europe, to asymmetric and very-high-speed digital subscriber loops (ADSL, VDSL).
In one example of a multicarrier system, before transmission, the available bandwidth is divided into parallel sub-bands(tones). The incoming data is distributed among all the available tones and used to modulate each tone. An Inverse Fast Fourier Transform operation converts the modulated tones into a time domain signal. Before entering the transmission channel, a cyclic prefix is added to the time sequence.
One reason for the popularity of MCM is the ease with which MCM can combat channel dispersion, provided the channel delay spread is not greater than the length of the cyclic prefix (CP). However, if the CP is not long enough, the orthogonality of the sub-carriers is lost and this causes both inter-carrier interference (ICI) and inter-symbol interference (ISI).
A technique for ameliorating the impact of an inadequate CP length is the use of a time-domain equalizer (TEQ) in the receiver. The TEQ is a filter that shortens the effective channel (by shortening the channel impulse response) to the length of the CP plus one.
Since transmission channels and noise statistics can change during operation, it is desirable to design an equalizer that changes when the receiver or received data changes. Such an equalizer is described as an adaptive equalizer. An adaptive equalizer design method is given in U.S. Pat. No. 5,285,474 (issued on Feb. 4, 1994 to J. Chow et al.). However, the algorithm of U.S. Pat. No. 5,285,474 requires training data. Similarly, the time domain equalizer described in U.S. Pat. No. 6,320,902 (issued on Nov. 20, 2001 to M. Nafie et al.) also requires training data.
It is also desirable to design an adaptive equalizer that does not require training data or identification of the channel. Such equalizers are described as blind adaptive equalizers. De Courville, et al. have proposed a blind, adaptive TEQ (M. de Courville, P. Duhamel, P. Madec, and J. Palicot, “Blind equalization of OFDM systems based on the minimization of a quadratic criterion,” in Proceedings of the Int. Conf. on Communications, Dallas, Tex., June 1996, pp. 1318-1321.) that relies on the presence of unused subcarriers within the transmission bandwidth. However, the method described by de Courville performs complete equalization rather than channel shortening. Since it is desired to perform channel shortening, the overall performance of an equalizer that performs complete equalization is expected to be worse.
There is a need for a blind adaptive equalizer designed for channel shortening.
It is therefore an object of this invention to provide methods for the design of a blind adaptive equalizer for channel shortening.
It is a further object of this invention to provide a blind adaptive equalizer for channel shortening.